A user is inactive if they are not interacting with the app. Once a user opens the app, they are considered active. As an example, a user who opens the app just to view the Leaderboard and never logs an action is considered active.
During a Team Challenge with average scoring, a team member will be removed from the Challenge (but not the app) if they remain inactive for 72 hours. This is to prevent team scores from being harmed by inactive members.